Tracking antenna system



Oct. 13, 1970 Filed May 1, 1969 w. KORVIN ETAL TRACKING ANTENNA SYSTEM 3Sheets-Sheet 1 CON lCb.L SCAN TRACK! NG DETECTORS DE CODE 2 EXUTAT \ONMATIZM nwavroes, lV/LL/AM Aaew/v MAM/(Mus 5 Q747qr MM ITIUP/VEYS' 1970w. KORVIN ETA!- 3,534,365

TRACKING ANTENNA SYSTEM Filed May 1, 1969 3 Sheets-Sheet 5 MAL/4M Kan/0vM1 ro/v KAI/14s M 41.7 If/OFNEKS United States Patent 19 Claims ABSTRACTOF THE DISCLOSURE Disclosed is a conically scanned antenna systemcomprising a multiplicity of wave guide feed elements arranged in anannular array for exciting a reflector system which generates asecondary radiation beam. The array includes elements extendingradially, as well as circumferentially. An excitation network isprovided whereby a plurality of elements is simultaneously excited. Thebeam is scanned both radially and circumferentially, by an excitationnetwork including means for removing excitation from some of theelements while applying excitation to adjacent previously unexcitedelements and elements which remain excited. In the center of the annulararray, there is provided a separate array for tracking to the reflectorboresight axis. The array can be mounted on a synchronous satellite oron a ground based radar.

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

The present invention relates generally to scanned antenna arrays andmore particularly to an electronically scanned antenna array including amultiplicity of elements arranged in an annulus.

One of the generally employed electronically scanned antenna systems isthe so-called phased array. In a phased array antenna, the antennaaperture comprises a multiplicity of radiating elements simultaneouslyexcited with a relative phase determinant of the desired direction angleof propagation relative to an antenna system boresight axis. Because theexcited elements occupy the entire aperture, phased array antennasinclude relatively massive electronic components that are heavy, complexand relatively ineflicient. Ineiticiency results from the attenuationproperties of phase shifters which must be included in the phased arrayexcitation network. The inclusion of phase shifters in the phased arrayexcitation network also reduces bandwidth of the antenna because of thewell known variation in phase that is introduced on differentfrequencies. The physical constraints on a phased array, relating tosize and weight, are such that a high gain phased array antenna can bemounted on existing spacecraft with only a great deal of difliculty, ifat all.

In our co-pending application, Ser. No. 759,256, filed Aug. 29, 1968.for Antenna Feed System and commonly assigned with the presentapplication, there is disclosed an antenna system for avoiding many ofthe problems associated with prior art phased array antenna systems. Insaid application, there is disclosed an antenna system capable ofsimultaneously deriving a plurality of steerable radiation beams byincluding, in combination with a secondary beam forming reflector, aprimary radiation source having several feed elements. To form a primarybeam, a plurality of adjacent feed elements is simultaneously in phaseexcited to the inclusion of other feed elements through a switchingmatrix. To scan the beam, excitation is removed from one of the feedelements and another, pre- 3,534,355 Patented Oct. 13, 1970 ice viouslyunexcited element adjacent the previously excited elements, is excitedby the switching matrix. Because the switching matrix includes only wideband devices, such as switches, circulators and hybrids, the frequencydependent problems encountered in phase arrays are obviated. Problemsconcerned with Weight and complexity are decreased because the excitedelements did not cover the entire aperture but form a primary radiationsource for a reflector system that scans a secondary radiation beam.

The antenna system disclosed in our previously mentioned co-pendingapplication is particularly adapted for tracking targets in the vicinityof an antenna boresight axis. While the principles disclosed in ourother application are applicable to wide angle acquisition functions, aswell as for tracking into an antenna system boresight axis, the arraydisclosed therein is generally limited by practical constraints toapproximately :12", 3 db on axis beamwidth from the antenna boresightaxis.

The present invention is an improvement on our previous invention,enabling a target to be acquired and tracked to angles within an annularregion extending from approximately :5 to i15 from an antenna boresightaxis. In combination with the system disclosed in our previousapplication, a target can be acquired at an oif axis angle of :12", 3 dbon axis beamwidth and tracked into the antenna boresight axis.

To attain the increased acquisition angle the present invention providesan annular array of substantially coplanar radiating elements forderiving a primary pattern broadside of the array, i.e., at right anglesto the plane of the array. The feed elements are arranged in a pluralityof rings having different radii and are circumferentially disposed aboutthe array. To provide secondary beams having the highest gain, theprimary radiating elements of the array are located in the focal surfaceof the reflector system. Thereby, the elements of the rings havingdifferent radii are stepped relative to each other, but each has acommon center along the boresight axis of the reflector system.

To scan the secondary radiation pattern derived from a reflector systemexcited by the feed array of the present invention, energy istransferred or switched between elements of the primary radiationsource. By sequentially switching the excitation between the variouselements of the radiation source, the secondary radiation beam isconically scanned in an annular pattern enabling wide angle acquisition,as well as directional information, of a target to be derived. Bydetermining the direction of a source with the acquisition array of thepresent invention, signals can be generated to control a centrallylocated tracking system of the type disclosed in our previouslymentioned co-pending application, enabling tracking of a target into theantenna system boresight axis. For targets located a great distance fromthe annular array of the present invention tracking is not generallyfeasible into the antenna system boresight axis because of the relativenull along the axis. The annuli radii cannot be extended into theantenna system boresight axis because of physical limitations on thepositioning and dimensions of the primary radiators. Hence, for longrange tracking into the boresight axis of an antenna system an array ofthe type disclosed in our previously mentioned co-pending application islocated centrally of the annular array of the present invention and bothderive broadside primary radiation beams for exciting a common reflectorsystem that derives a secondary beam illuminating an area having arelatively wide angle.

In accordance with one preferred embodiment of the present invention,the primary radiation feed elements are abutting rectangular waveguidesor horns excited with linearly polarized energy such that the E fieldgenerated by each element extends circumferentially, while the H fieldextends radially. A plurality of radiating elements are simultaneously,in phase excited to derive a primary beam for exciting the reflectorsystem that generates the secondary beam. To switch the secondaryradiation beam with a low crossover level between adjacent beams so thatthe beam position appears to be smoothly transferred, the switchingmatrix is activated so that energy is removed from only some of theexcited elements while excitation is applied to previously unexcitedelements adjacent the elements which remain excited. The position of thesecondary radiation pattern can be translated both radially andcircumferentially by controlling the excitation of the primary radiationelements between the several rings and about the circumference of therings.

One application of the present invention involves monitoring theposition of low orbiting spacecraft. In such an application, an antennaarray in accordance with the present invention is mounted on asynchronous satellite to scan an annular region about the periphery ofthe earth. In another application, the trajectory of a rocket can betracked with a ground station radar.

In either of these applications, there is a possibility of jamming toprevent acquisition in a predetermined portion of the area illuminatedby the antenna array. Jamming, or other noise, generally subsists onlyover a relatively small percentage of the entire region which the arrayof the present invention is capable of covering. In accordance with afeature of the invention, a noise or jamming signal in a relativelysmall region can be eliminated from the response of a receiver connectedto the array by changing a connection to the receiver without affectingthe remainder of the excitation network or array.

Another feature of the invention is that the array and excitationnetwork are wide band and constructed so that a number of diversefrequencies can be simultaneously transmitted and received. This resultis achieved by utilizing wide band components having the same relativephase shift on each frequency component so that the feed elements are inphase excited at the same frequency.

It is, accordingly, an object of the present invention to provide a newand improved antenna array, particularly adapted for acquisition andtracking purposes.

Another object of the present invention is to provide a new and improvedwide band antenna array for electronically generating a conicallyscanned beam.

Another object of the invention is to provide a new and improvedelectronically scanned antenna array having a relatively small number ofcomponents so that it is lightweight and not particularly complex andhas high efficiency, thereby being susceptible to use on spacecraft.

Another object of the present invention is to provide a system fortracking targets with an antenna having the ability to acquire andlocate the direction angle of the target over a wide angle and into anantenna boresight axis.

Another object of the present invention is to provide a new and improvedradiation array for deriving a conically scanned radiation pattern Withan electronic switching network.

Still another object of the invention is to provide a tracking antennasystem including an array of radiator elements wherein a selectedsegment of the area normally covered by the radiation beam produced bythe elements can be blocked at will so that noise and other interferencein a relatively small area are not fed into a receiver.

A further object of the invention is to provide a new and improved wideband antenna array capable of deriving a plurality of conically scannedradiation patterns simultaneously at diverse frequencies.

The above and still further objects, features and advantages of thepresent invention will become apparent 4 upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a side sectional view of one embodiment of the feed of thepresent invention in combination with a reflector system for deriving asecondary radiation pattern;

FIG. 2 is a plan view of the feed illustrated in FIG. l;

FIG. 3 is a circuit diagram of the excitation network for driving thefeed array of FIGS. 1 and 2;

FIG. 4 is a block diagram of a system adapted to utilize the feedillustrated in FIGS. 1 and 2;

FIG. 5 is a perspective view of the earth and a satellite carrying thefeed of the present invention; and

FIG. 6 is a top View of the earth illustrating the manner by which theannular feed of the present invention provides coverage for areasextending beyond the surface of the earth.

Reference is now made specifically to FIG. I wherein there isillustrated a side sectional view of parabolic reflector 11, describedas a surface of revolution about boresight axis 12 and having an apexlocated at the intersection of axes 12 and 13, the latter being at rightangles with the former. Positioned in the focal surface of parabolicreflector 11 is primary feed array 14, including linear array 15,preferably including a plurality of abutting horn feed elements asdescribed in our aforementioned copending application, and annular array16. Array 16 is concentric with array 15, each having a centercoincident with the boresight axis 12 of parabolic reflector 11 andpositioned to direct energy broadside toward the reflector.

Array 16 includes three annuli 17-19, each coaxial wih boresight axis12, but having a different radius from the boresight axis. The positionof rings 17-19 from axis 13 of reflector 11 increases for rings havinggreater radii in such a manner that each ring is substantiallycoincident with a parabolic focal surface 21 for reflector 11.Similarly, the elements of array 15 are displaced from axis 13 so thatthey lie along parabolic focal surface 21, as described in detail in theaforementioned copending application. By positioning the feed elementsof arrays 15 and 16 so that they lie along parabolic focal surface 21the secondary beam reflected from reflector 11 has relatively lowamplitude side lobes compared to the amplitude of the main lobe. WhileFIG. 1 illustrates the primary radiation source, feed 14, as beingpositioned in front of parabolic reflector 11, it is understood that theprimary radiation pattern forming array can be replaced with asubreflector excited by an array positioned in the vicinity of the apexof reflector 11, in accordance with the wellknown Cassegrain assembly.

As specifically illustrated in FIG. 2, each of rings 17-19 includes amultiplicity of abutting horns or waveguides having substantiallyrectangular feed apertures, each of which subtends an arc of the samelength about boresight axis 12. In the specifically illustrated array,each ring includes 48 horns or waveguide elements, with the elements ofthe innermost ring 17 being -147, the elements of the intermediate ring18 being 200-247 and the elements of the outermost ring 19 being300-347. Each of the elements in array 16 is excited to the TE mode,with the E plane being directed circumferentially of the array, asindicated by the arrows in feed elements 132, 133, 232, 233, 332, 333,106, 206 and 306.

The radial dimension of each of the elements is long enough to support ahalf wavelength of the H field of the frequency driving the array whilethe circumferential dimension is preferably a fraction of a halfwavelength of this frequency. By selecting the circumferential dimensionof each array as a fraction of a half wavelength, a smooth transition inswitching between elements can be achieved by simultaneously exciting asufficient number of elements. To provide a secondary radiation beamhaving a narrow illumination field at any instant so that accurateposition tracking can be attained, the total number of elementssimultaneously excited is a relatively small percentage of the totalangular coverage of the array. In the specifically illustratedembodiment, a pair of circumferential elements in each of two annuli issimultaneously excited so that the total number of simultaneouslyexcited elements is four. It is to be understood, however, that for.smoother transitions and lower amplitude crossover amplitudes betweenadjacent secondary beams, the number of elements simultaneously excitedcan be increased as desired.

To switch the position of the beam, in either a circumferential orradial direction, first and second adjacent, previously excited elementsare deactivated, third and fourth elements remain excited, while fifthand sixth elements, adjacent the third and fourth elements, areenergized with the same phase as the third and fourth elements. Forexample, in the array of FIG. 2, feed elements 232, 233, 332 and 333 arein phase excited so that a beam having a phase center at theintersection point of these elements is generated. To scan the beamcircumferentially in the clockwise direction, elements 232 and 332 aredeactivated, elements 234 and 334 are switched to an excited state andelements 233 and 333 remain in an excited state. As the primary beamphase center is switched about the periphery of ring 18 in response toswitching between the circumferentially disposed elements an annular,conically scanned secondary radiation pattern is generated by reflectorsystem 11. At distances quite remote from the antenna, there is asubstantial void region in the annular secondary radiation pattern. Toswitch the beam radially toward boresight axis 12, excitation is removedfrom elements 332 and 333, elements 132 and 133 are switched to anexcited state and no change in the excitation of elements 232 and 233occurs so that elements 132, 133, 232 and 233 are in phase excited. Byswitching the radiation phase center inwardly from the intersection ofrings 18 and 19 to the intersection of rings 17 and 18 the void regionin the annular radiation pattern is reduced, as is the outer coverageangle. At both radial positions, however, there is sometimes asuflicient null in the center of the annular secondary pattern toprevent tracking of a target to boresight axis 12.

To enable a target to be acquired and tracked over an extremely wideangle and into boresight axis 12 of parabolic reflector 11, linear array15 is mounted interiorly of the inner radius of ring 17, with which itis coaxial. To provide tracking into boresight axis 12 along any radial,linear array 15 is rotated in response to the outputs of the elements ofannular array 16 to align the linear array waveguide elements with theelements of ring 17 which indicate the angular position of a trackedtarget. To enable linear array 15 to illuminate a region havingsubstantially the same arc as that illuminated by the simultaneouslyexcited elements of annular array 16, each of elements 15 has an H planeaperture dimension approximately equal to the boundary between oppositeradial edges of the excited region of array 16. For example, in FIG. 2,the H plane aperture dimension of each of the elements in linear array15 equals the sum of the sides of feeds 132 and 133 in the E planedirection of the latter horns. It is noted by reference to ourpreviously mentioned application that the E fields of elements 15 extendradially along the array, so that they are orthogonal to the E fields ofannular array 16. Therefore, it is preferable to employ the trackingsystem of the present invention in combination with targets including atransponder capable of transmitting and receiving circularly polarizedelectromagnetic waves.

Reference is now made to FIG. 3 of the drawings wherein there isillustrated a circuit diagram of the network for exciting the annularfeed array of FIGS. 1 and 2. In the circuit diagram of FIG. 3, only thatportion of an excitation matrix for exciting feed elements 132-134,232234 and 332334 with a receive frequency feeding receiver 431 isspecifically illustrated, with the remainder of the circuitry beingdesignated by phantom lines. Each of the feed elements of FIG. 3 isexcited by a pair of different frequencies in the receive mode and astill further pair of diverse frequencies in the transmit mode. The fourdifferent frequencies exciting the feed elements of FIG. 3 can energizeeach of the feed-s simultaneously because of the frequency multiplexingarrangement provided. To simplify the circuit of FIG. 3, the excitationmatrix 409 for only one frequency, in the receive mode, is specificallyillustrated, with the matrices for both transmit frequencies and theother receive frequency being shown by boxes 404, 405 and 408.Excitation matrix 408 for the other receive frequency is the same asthat specifically illustrated, while the identical transmit excitationmatrices 404 and 405 differ from receive matrix 409 only insofar as isrequired to enable energy to propagate to, rather than from, the feeds.

Considering now the excitation network shown in FIG. 3, each of theelements is excited through a fixed circulator 401, having an input port402 responsive to the outputs of frequency multiplexer or diplexer 403which is in turn driven by two different transmit frequencies generatedby matrices 404 and 405. Fixed circulator 401 includes an outputterminal 406 for driving frequency multiplexer 407, which in turn feedsdiverse frequencies to receive matrices 408 and 409. Fixed circulator401 includes an additional port 411 for coupling energy from port 402 tothe feed element and for coupling energy from the feed element to thecirculator output port 406.

Consideration is now given to the specific apparatus included wthinmatrix 409. In the drawing, matrix 409 is arranged to include three rows412, 413 and 414 of latching circulators, each driven by an output of adifferent one of frequency multiplexers 407. Adjacent output ports ofadjacent circulators in each of rows 412414 are connected to input portsof hybrids connected in three separate rows 415-417. The sum outputports of the hybrids in row 416 are connected to input ports of latchingcirculators in an additional row 418. The latching circulators in row418 selectively couple energy into input ports of hybrids in rows 419and 420. The hybrids of row 419 are also responsive to energy fed tooutput ports of hybrids in row 415, while the hybrids of a further row420 are responsive to energy at the output ports of hybrids in row 417.The connections to the hybrids in rows 419 and 420 are such that theenergy derived from one port of each hybrid is equal to the sum of theenergies transduced by the four feeds connected to the hybrid so thatthe four feeds are in phase excited. The other output port of eachhybrid of rows 419 and 420 derives an output equal to the sum of theenergies from an adjacent pair of circumferential elements in eitherrings 18 or 19 minus the sum of the energy from a second pair ofadjacent circumferential feed elements in either rings 17 or 18. One ofthe output ports of each of the hybrids in rows 419 and 420 is connectedto receiver 431, in a manner described infra, so that the receiver cantrack signals in the pattern generated by array 16.

The latching circulators in rows 412, 413, 414 and 418 are responsive tobipolarity voltages to control the direction of energy flow from thecirculator input terminal to one or the other of the circulator outputterminals. If a zero level voltage is applied to the latchingcirculators, they remain in their present state. If a positive levelvoltage is applied to the latching circulators, they assume a specificdirection of energy circulation. If a negative level voltage is appliedto the latching circulators, they reverse the direction of energycirculation. The bipolarity signals are applied to the latchingcirculators in such a manner as to enable four adjacent feed elements tobe simultaneously activated. To switch the phase center of the energytransmitted from the annular array, 21 different set of control voltagesis applied to the latching circulators of rows 412, 413, 414 and 418 totransfer the excitation pattern as described supra.

Consideration will now be given to the specific circuitry connectingreceiver 431 to feed elements 233, 234, 333 and 334. As indicated supra,each of feed elements 233, 234, 333 and 334 is connected through adifferent fixed circulator 401 and frequency multiplexer 407 to adifferent latching circulator. In particular, feed elements 233, 234,333 and 334 drive the input ports of latching circulators 432, 433, 434and 435, respectively. Energy derived from the right output ports 436and 437 of latching circulators 433 and 435 is applied to input ports ofhybrids 438 and 439, respectively, while the left-hand output ports ofcirculators 432 and 434 are connected to additional input ports ofhybrids 438 and 439. The difference output ports of hybrids 438 and 439are connected to different matching load resistors 441, while the sumoutput terminals of hybrids 438 and 439 are respectively applied to theinput port of latching circulator 442 in row 418 and an input terminalof hybrid 443 in row 419.

Output terminal 444 of latching circulator 442 is connected to the otherinput port of hybrid 443, the sum output port of which drives receiver431 through reversing switch 445. The difference output port of hybrid443 is connected through reversing switch 445 to match ing load resistor446. In normal operation, reversing switch 445 is connected in theposition indicated. If, however, it is desired to decouple the beamassociated with feeds 233, 234, 333 and 334 from receiver 431 becausenoise due, for example to jamming or other interference, is extant inthe region illuminated by the pattern resulting from excitation of thefeeds, reversing switch 445 is activated so that the difference outputport of hybrid 443 is connected to receiver 431 and the sum output portdrives load resistor 446. Thereby, a null output is derived from thereceiver 431 with reversing switch 445 activated to the oppositeposition from that illustrated.

Because of the similarity between the feed elements and receivers forthe remainder of the system, no detailed description of the connectionsfor the remaining circuit elements is provided.

To consider the operation of the matrix illustrated by FIG. 3, assumeinitially that latching circulators 432 and 434 are activated totransfer energy in the counterclockwise direction while circulators 433,435 and 442 are energized to transfer energy in the clockwise direction.Under these conditions, feeds 233, 234, 333 and 334 are connected to bein phase excited by receiver 431, whereby the phase center of radiationderived by the feeds is at the geometric center of the feeds,approximately co incident with the common intersection point thereof.Energy is coupled from feeds 333 and 334 to receiver 431 via thecounterclockwise and clockwise propagation paths of latching circulators434 and 435, respectively. The left and right output ports of latchingcirculators 434 and 435 feed energy into hybrid 439, which drives oneinput of hybrid 443 with the sum of the energies transduced by elements333 and 334. The other input to hybrid 443 is responsive to output port444 of circulator 47, which is in turn responsive to the sum of theenergies transduced by elements 233 and 234, as coupled through thecounterclockwise and clockwise propagation paths of latching circulators432 and 433 to hybrid 438. The energy from latching circulator 442 issummed with the energy at the output port of hybrid 439 and coupled toreceiver 431.

To scan the radiation beam of annular array 16 in the counterclockwisedirection so that the pattern phase center is transferred from theintersection point of feed elements 233, 234, 333 and 334 to theintersection point of feeds 232, 233, 332 and 333, latching circulators432 and 453 are activated to propagate energy in the clockwise directionand latching circulators 451 and 452, responsive to energy transduced byfeed elements 232 and 332, are activated to propagate energy in thecounterclockwise direction. Latching circulator 453 is responsive to thesum of the energy transduced by feeds 232 and 233, as coupled from theright and left output ports of latching circulators 432 and 451 throughhybrid 454. The energy at output terminal 455 of latching circulator 453is summed with the energies transduoed by feeds 333 and 332, as coupledthrough latching circulators 434 and 452 to hybrid 456 which in turndrives an input of hybrid 457.

From the foregoing description, it is believed evident as to the mannerby which the phase center of the radiation derived from rings 18 and 19of array 16 is stepped between adjacent beam positions with a relativelylow amplitude cross-over determined by the circumferential spacing ofthe elements in the rings. To shift the phase center radially of array16 from the intersection of annuli 18 and 19 to the intersection ofrings 17 and 18, the latching circulators of row 414 are activated topropagate energy in either the clockwise or counterclockwise direction.In adidtion, the circulators of row 418 are selectively activated toenable energy to be propagated in the counterclockwise direction fromthe output ports of the hybrids in row 416 to the hybrids in row 420.

For certain applications relating to tracking a target which feedsenergy of the frequency coupled to the lefthand output port of frequencymultiplexers 407 into matrix 409, the terminal of each reversing switch445 connected to receiver 431 is connected through an isolation network461, which may be a unilaterally conducting amplifier to detectingnetwork 462. In response to the input signal to detector 462 exceeding apredetermined level for a certain time period, the detector derives abinary output signal indicating that a target is in the regionilluminated by the excited radiators of array 16. Thereby, an indicationof the location of a target can be ascertained from the conicallyscanned beam derived from array 16.

To enable the position indicating information derived from detectors 462to be utilized in combination with the boresight axis illuminating array15, the system of FIG. 4 is provided. In the system of FIG. 4, theoutput of conically scanned tracking detectors 462, one of which isprovided for each of the circumferential positions of the array 16, isrepresented by network 501. The circumferential poistion indicatingbinary signals derived from network 501 are fed to decoder 502 whichderives an analog signal indicative of the angular position of a sourceilluminating the annular array 16 of FIGS. 1 and 2. The analog output ofdecoder 502 is fed to servo network 503, having a mechanical shaftoutput 504 for rotating array 15 about boresight axis 12 of parabolicreflector 11. Array 15 is rotated in response to the output of network501 so that it is positioned along a radial coincident with the radialpath of a target being tracked by the conically scanned radiationpattern generated by array 16.

Once a target has been tracked to the inner radius of fixedly positionedarray 16, linear array 15 tracks it into the antenna system boresightaxis 12. The elements of linear array 15 are excited by matrix 505, ofthe type described in our conpending application previously mentioned,which also controls the position of the linear array. While a lineararray is specifically illustrated for illuminating the region within theannular radiation beam pattern of array 16, it is to be understood thata planar array can also be utilized, as disclosed in the copendingpreviously mentioned application.

One application of the present invention, relating to monitoring theposition of low orbiting spacecraft S10, is illustrated in FIGS. 5 and6. In FIG. 5, a synchronous satellite 511 is positioned in space above afixed subsatellite point on earth, as is achieved by causing thesatellite to rotate at the same velocity as the earth at an altitude ofapproximately 23,000 miles. On synchronous satellite 511 there ismounted an array and excitation network, of the type illustrated byFIGS. 1-3. The annular array 16 of FIGS. 1 and 2 conically scans theearth to provide an annular illumination pattern 512, extending fromapproximately :5 to $12.5 degrees from the antenna system boresightaxis, which is usually coincident with the local vertical of thesynchronous satellite. To enable a region 513 within the core of theannulus 512 to be illuminated, linear array 15, or its planarequivalent, is provided. To track objects very close to the surface ofthe earth, or objects moving into proximity with the inner illuminationboundary of annulus 512, annuli 17 and 18 are activated to the exclusionof annulus 19 to generate a secondary radiation beam 514. For objects ata higher altitude from the earths surface and displaced from thesatellite local vertical by more than approximately :9 degrees, annuli18 and 19 are excited to the exclusion of annulus 17 to derive secondarybeam 515. By utilizing the present inven tion, therefore, a relativelywide portion of the earth can be illuminated and the shadow zone of theantenna system of satellite S11 is confined only to that portion of theearth which is blocked and cannot be in line of sight communication.

For certain applications, there is no need to include a central array incombination with the annular array of FIGS. 1 and '2. Typical examplesof such applications are relatively short range radar tracking systemswherein there is suflicient energy down the boresight axis of thereflector system from the inner ring 17 of the annular array to enable atarget, such as a missile, to be accurately tracked. The position of aclose target tracked on the boresight can be often ascertained withsufficient accuracy by conically scanning the inner pattern of array 16.As the target moves away from the antenna system boresight axis theannular array is activated so that exterior pattern is excited.

While there has been described and illustrated one specific embodimentof the invention, it will be clear that variations in the details of theembodiment specifically illustrated and described may be made withoutdeparting from the true spirit and scope of the invention as defined inthe appended claims. For example, if it is desired to transmit and/orreceive more than two diflerent frequencies simultaneously, thefrequency multiplexers, illustrated as diplexers in FIG. 3, can bereplaced with triplexers, quadriplexers, etc. Also, the feed elementscan be excited with circularly polarized energy, rather than linear,with an accompanying increase in crossover level because only oneelement can be excited at a time.

We claim:

1. An array for generating a primary radiation beam pattern forilluminating a reflector system that derives a secondary radiation beam,said system having a boresight axis, comprising several radiatingelements arranged in a plurality of annuli, each of said annuli having adifferent radius and a common axis coincident with said boresight axis,each of said annuli including a multiplicity of circumferentiallydisposed ones of said elements, and means for exciting only a selectednumber of said elements at the same time for deriving a primaryradiation beam from combinations of the selected number of saidelements.

2. The system of claim 1 wherein said exciting means includes means forexciting and deactivating adjacent ones of said elements to enable aconically scanned secondary beam to be derived.

3. The system of claim 1 wherein said reflector system has an apex and asecond axis transverse to said boresight axis and intersecting saidapex, each of said annuli being displaced from said second axis by adifferent distance so that it lies in a focal surface of said reflectorsystem.

4. The system of claim 1 wherein the secondary radiation patternresulting from excitation of said annular array is annular in shape andfurther including a second array of radiating elements extending throughthe boresight axis and into proximity with the annuli having theshortest radius, said second array illuminating said reflector to derivean additional secondary beam pattern within the inner radius of thesecondary beam pattern derived from the annular array.

5. The array of claim 1 further including a receiver responsive to saidexciting means, and said exciting means includes means for at willdecoupling said receiver from any of the selected number of saidelements.

6. The array of claim 1 further including transmitter and receivermeans, and said exciting means includes means for simultaneouslycoupling energy between said elements and both said transmitter andreceiver means.

7. The array of claim 1 wherein said exciting means includes frequencymultiplexing means for simultaneously exciting said elements withdifferent frequencies.

8. An array for generating a primary radiation beam pattern forilluminating a reflector system that derives a secondary radiation beam,said system comprising several radiating elements arranged in aplurality of annuli, each of said annuli having a dilferent radius and acommon axis coincident with said boresight axis, each of said annuliincluding a multiplicity of circumferentially disposed ones of saidelements, and means for selectively in phase exciting a plurality ofadjacent ones of said elements simultaneously.

9. The system of claim 8 wherein said exciting means includes means forsimultaneously in phase exciting a plurality of circumferentiallyadjacent ones of said elements with linearly polarized energy.

10. The system of claim 9 wherein said exciting means includes means forswitching the excitation of said elements so that at least one elementof said plurality remains excited while at least another element of saidplurality is deactivated and at least one previously deactivated elementis excited, said at least one previously deactivated element beingadjacent to an element remaining excited.

11. The system of claim 8 wherein at least three annuli are provided,and said exciting means includes means for simultaneously in phaseexciting with linearly polarized energy a plurality of adjacent ones ofsaid elements on different annuli, said plurality being less than thenumber of said annuli.

12. The system of claim 11 wherein said exciting means includes meansfor simultaneously in phase exciting a plurality of circumferentiallyadjacent ones of said elements with linearly polarized energy.

13. The system of claim 8 wherein at least three annuli are provided,and said exciting means includes means for simultaneously in phaseexciting with linearly polarized energy a plurality of circumferentiallyadjacent ones of said elements and a plurality of adjacent ones of saidelements on different annuli, said plurality being less than the numberof said annuli.

14. The system of claim 13 wherein said exciting means includes meansfor circumferentially and radially switching the excitation of saidelements so that at least one element of said plurality remains excitedwhile at least another element of said plurality is deactivated and atleast one previously deactivated element is excited, said at least onepreviously deactivated element being adjacent to an element remainingexcited.

15. A system for generating secondary radiation beams from a reflectorcomprising a first feed illuminating said reflector for deriving anannular secondary radiation pattern, and a second feed illuminating saidreflector for deriving a second radiation pattern steerable within theinner radius of the annular pattern.

16. The system of claim 15 wherein said annular pattern is conicallyscanned.

17. The system of claim 15 further including means for controlling theposition of the second pattern in response to a signal derived from thefirst feed.

18. A radiating feed array comprising several radiating waveguideelements arranged in a plurality of annuli, each of said annuli having adilferent radius and a common axis, each of said annuli including amultiplicity of circumferentially disposed ones of said elements, saidelements being positioned to excite a radiation beam broadside of theannuli and in the general direction of said axis.

19. The array of claim 18 wherein the elements in each of said annuliare coplanar at right angles to the axis and the planes of the differentannuli are stepped relative to each other in a direction extending alongthe axis References Cited UNITED STATES PATENTS Gitzendanner 343-400 US.Cl. X.R.

